It's by turns annoying and amusing: the way people on the left complain that orthodox economics has gone off the rails, and that we need fresh thinking to inform government policies. Apart from coming up with a lot of age-old tropes that economic models do not mimic real behaviour, or take account of information asymmetries, the main item of evidence is the persistance of austerity policies in the developed world.

But the main critics of austerity turn out to be.... orthodox economists. People like Joe Stiglitz, Paul Krugman and Martin Wolf. And newspapers struggle to find economists to make the case for the defence. The Financial Times often resorts to Niall Ferguson, who is a historian, not an economist, and no match for a Nobel laureate like Mr Krugman. The British Labour party is even roping in economics professors to bolster its economic credibility.

In fact there is a brand of orthodox pro-austerity economists. These are the old "supply-siders" from such institutions as the Chicago Business School, who developed a line of "neoclassical" economics, and rebelled against what was once the Keynesian orthodoxy. This branch of thinking grew out to the economic crisis of the 1970s, but proved utterly useless when the crisis of 2007/08 hit. Neoclassical economists pipe up here and there in America, but are mostly silent, their credibility shot-through. That leaves the field nearly unchallenged for the neo-Keynesians - at least far as the public debate in newspaper columns is concerned, in Britain, anyway.

Which leaves us with a mystery. Why are governments, from Europe to America (though not Japan, interestingly), ignoring the orthodox economists? Two explanations are usually offered by their critics. One is rank incompetence or wilful blindness. The other is a political agenda that austerity plays to, usually involving making the rich richer. Neither explanation stands up to close examination.

I am wary of accusations of incompetence, especially when made about clearly intelligent people, such as most politicians and technocrats involved in government finance. This is something I learnt as a history undergraduate (I studied both science and history in my original undergraduate incarnation, long before my study of economics as a mature student). Such accusations are bandied about freely down the ages, but never stand up to scrutiny. Mostly the wilful blindness comes from the people making the accusation, who cannot entertain the idea that there is a rival point of view to their own. Modern economic policy is no exception.

The political agenda is a bit more plausible. Perhaps governments are in hoc to big business interests and those of the wealthy? But if the last 150 years of history has taught us anything, it is that if poorer members of society are prospering, the rich will prosper also, and be left in peace. This is even more true of big corporate interests than anybody else. It is harder to make money in a stagnant economy. Those malign influences are there in politics, but their effects are altogether more subtle than doing down poor people to help line the pockets of the rich.

Sensing that these explanations don't work, many on the left build up an idea of "neoliberalism". This is a philosophy based on the old supply-side or neoclassical economics that may be waning in academic economics, but still holds a grip on the lesser mortals who staff finance ministries and banks, and other parts of the "elite". But this too is inadequate as an explanation. Certainly it is possible to identify a series of beliefs and biases amongst policymakers that equate to economic liberalism. But they do not explain austerity as a macroeconomic policy. And besides, we need to understand why the hold of these beliefs is so strong. Clearly some on the left think that an outdated economic orthodoxy is to blame. But surely such theoretical constructs cannot by themselves have such a grip on so many intelligent and practical minds?

Instead of a conflict between different types of theory, what is really going on is a conflict between theory and practice. The theoreticians may be gung-ho about fiscal and monetary stimulus, but the people who implement policy are acutely aware of the practical problems and risks. There are three particular practical issues about which the theoreticians are dismissive, but which weigh heavily on the practical types: economic efficiency; public investment; and financial markets.

First take economic efficiency. Pretty much everybody agrees that, ultimately, living standards depend on economic efficiency, or productivity. This piece of orthodoxy could be challenged, but that is not what most on the left mean (traditional Greens being the exception, along with liberal voices in the wilderness like mine) when they call for fresh thinking. They see slow economic growth as a sign of failure as much as any conservative does; and that ultimately is based on productivity. But economic efficiency is hard work politically. Both businesses and workers like to protect their patches with taxes, government agencies and regulations that keep the winds of change at bay. This is especially the case in Europe and Japan. And yet, in order to achieve long-term growth, these vested interested must be tackled, and reforms enacted. This has been shown in countless contexts in both developed and developing world. Mostly reforms have an economically liberal character - but only because this approach genuinely unlocks long-term efficiency. Far-sighted politicians and officials want to use every possible chance to advance reforms. That includes the pressures created by economic hard times. Theoretical economists might suggest that boom years are the best time to push through reforms, or that reforms can be covered by macroeconomic leniency. Politicians know that the opposite is the case - it too difficult to muster the political imperative in easy times, or if short-term macroeconomic policies take the heat off.

Reform and austerity are not necessarily the same thing, but they almost always are. This debate, of course, dominates discourse in the Euro zone, where economic hardship is concentrated in less efficient economies. Critics of austerity there offer no way forward for improved efficiency, beyond the hope that public infrastructure investment will deliver the growth they seek.

Which brings us to the difficulties of public investment. To theoretical economists this is the magic bullet. Public investment in infrastructure both yields gains to long-term efficiency, and a short term fiscal stimulus. The economists are exasperated that so few governments seem to follow their advice. And yet public investment is a graveyard of roads to nowhere and white elephants. When the imperative to invest is political, the choice of project becomes political too. It is very hard to make sensible choices. China was much lauded for its infrastructure investment programme following the crash. This has now turned into a major headache, as so much of the money was wasted on empty cities and useless infrastructure. Something similar happened in Japan in the 1990s. Finance ministry officials are rightly wary.

And then there are the financial markets. If I've heard one economist here in Britain suggest that now is a fabulous time for the government to borrow, or even "print", money, I've heard it from a hundred. With so much demand for government bonds in the markets, and inflation looking mortally wounded, just what are you worrying about? But none of these economists work at the sharp end of government finance. If they did, such sanguinity would remind them of the sort of thinking that got the world's banks into the disaster in the first place: a reckless confidence that markets would behave in future as they do now.

Alas life is much more complicated than that. Grounds for confidence in the financial markets is stronger in some places than others. Japan has a massive export industry that sees to all its foreign currency needs, so that the state can borrow and even print the Yen with reasonable confidence. Which is what it has been doing, in prodigious quantities, for the last two decades, although to little apparent effect. The US is another country that can feel reasonably secure, even though its balance of trade is less benign than Japan's. The dollar is the world's de facto reserve currency. The United Kingdom, however, shares neither of these strengths. It needs to draw on overseas institutions and businesses, and its own private sector, in order to finance its significant current account and trade imbalances. This is not a problem that printing the Pound can help with. The state has been extraordinarily adept at handling this risk over the last few decades. But that is because of the conservatism that is currently attracting so much criticism.

To me the theoretical economists, the practical policymakers, and most of their leftist critics are all trapped by an orthodox way of looking at the world through economic aggregate statistics. This means that they are failing to take on the deeper problems that society faces: economic and environmental sustainability, alienation, and the gravitation of wealth to successful people and places. That has very little to do with the politics of austerity. People on the left who call for fresh thinking should be careful what they wish for.